Handling video transition errors in video on demand streams

ABSTRACT

A system, method, and apparatus for handling transition errors is presented herein. The transition errors include handling unreported time base discontinuities during trick mode transition, miscalculated time stamps during trick mode transition, erroneous sequence end codes, and unreported broken group of picture transmission. Unreported time base discontinuities are detected by comparing the program clock reference (PCR) value of the data packet to the system time clock (STC). If the difference exceeds a predetermined threshold, the STC is set to the PCR value. Miscalculated time stamps are detected by examining the difference in PTS values between temporally adjacent data packets. If the difference is not within a margin of error from predetermined value, the PTS is disabled. Unreported broken groups of pictures are handled by skipping the first two B-frames of the first group of pictures following a trick mode transition. Erroneous sequence end codes are detected by determining whether a packet containing a sequence end code is associated with a time base change. If the packet is not associated with a time base change, the sequence end code is disregarded.

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BACKGROUND OF THE INVENTION

The present invention is directed to digital video, and moreparticularly to handling video transition errors in video-on-demandstreams.

Video-on-demand is a service which provides customers with the abilityto select movies when they desire. The movie is then transmitted from avideo rental business in combination with a video server. Utilizingcurrent technologies digital video-on-demand servers usually deliverMPEG-2 transport streams for interactive services such as Pay-per-view,Quasi video-on-demand services, near video-on-demand services, and truevideo-on-demand services.

Pay-per-view (PPV) services are services which the user signs up andpays for specific programming, similar to existing cable television PPVservices. Quasi video-on-demand services are services in which users aregrouped based on a threshold of interest. Users can perform at thetemporal control activities by switching to a different group. Nearvideo-on-demand are services in which functions like forward and reverseare simulated by transition in discrete time intervals, normally on theorder of five minutes. Such capabilities are provided by multiplechannels with the same programming skewed in time. True video-on-demandservices are services in which the user has complete control over thesession presentation. The full-function VCR capabilities, includingforward and reverse play, freeze, and random positioning. Truevideo-on-demand needs only a single channel per customer.

To achieve VCR type capabilities, known as trick modes, a set of filesfor various tasks modes are generated in advance by preprocessing theMPEG data in the video-on-demand servers. For example, the fast forwardfile is generated by transmission of the I-frames only. The I-frames aretransmitted at a frame rate of 10 frames/sec. When playing the fastforward file, although the frame rate is lower, the data rate is thesame because I-frames are associated with a greater amount of data.

When a user selects a trick mode, the time base is likely to change. Forexample, during a fast forward, another video elementary stream istransmitted which is associated with another time base. Duringtransition between the regular play mode and a trick mode, or viceversa, the video server should indicate the discontinuity in the timebase to the decoder. Additionally, the video server must calculate timestamps for the other video elementary stream using the other time base.Additionally, after some transitions, a new video elementary stream iscommenced starting from a particular picture, such as a return to normalplay from pause. If the picture is in the middle of a group of pictures,the video server should indicate the foregoing to the decoder.

However, many preexisting video servers are not equipped to properlyhandle trick mode transitions. Consequently, the decoders generate anumber of errors during trick mode transitions.

Accordingly, it would be advantageous if techniques for handling videotransition errors are provided. Further limitations and disadvantages ofconventional and traditional approaches will become apparent to one ofskill in the art, through comparison of such systems with embodimentspresented in the remainder of the present application with references tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A system(s), method(s), and apparatus(es) for handling video transitionerrors in video-on-demand streams are presented herein. The videotransition errors often occur because of erroneous operation of thevideo server.

In one embodiment, a decoder is configured to handle unmarked time basediscontinuities. Time stamps for incoming packets are compared to asystem time clock. In the case where the system time clock and the timestamp differ by more than a predetermined threshold, the system timeclock is changed to the time stamp of the data packet. When the systemtime clock is changed, the video decoder automatically disables timemanagement and decodes packets received prior to the system time clockchange using the vertical synchronization pulse for synchronization.After the packets received prior to the system time clock change aredecoded, the time management is enabled using the changed system clockvalue.

In another embodiment, a decoder is configured to handle unmarked timebase discontinuities between an original time base and a new time baseby calculating time stamps using the original time base. The time stampsbased on new time base are replaced by the calculated time stamps usingthe original time base.

In another embodiment, miscalculated time stamps are detected when filmcoded pictures are received after a transition for display using theNTSC standard. The difference between the time stamps of an instantpicture and a previous picture is calculated and compared, either to thetime length for two fields, or three fields. The difference is comparedto the time length for three fields, wherein the previous frame includesa repeat_first_field parameter. Otherwise, the difference is compared tothe time length for two fields. In the case where the difference is notwithin a predetermined margin of error from the compared time length,the time stamp for the instant packet is disabled.

In another embodiment, miscalculated time stamps are detected when filmcoded pictures are received after a transition for display using the PALstandard. The difference of time stamps between an instant picture and aprevious picture is compared to the time length for two fields. In thecase where the difference is not within a predetermined margin of errorfrom the compared time length, the time stamp for the instant packet isdisabled.

In another embodiment, receipt of pictures in the middle of a group ofpictures is handled by always skipping the first two B-pictures receivedafter a transition.

In another embodiment, erroneous sequence end codes are detected bydetermining if the sequence end code is accompanied by a time basechange. If the sequence end code is not accompanied by a time basechange, the sequence end code is determined to be erroneous anddisregarded.

These and other advantages and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary video distribution systemwherein the present invention can be practiced;

FIG. 2A is a block diagram of an exemplary video stream;

FIG. 2B is a block diagram of exemplary encoded frames;

FIG. 2C is a block diagram of encoded frames in data dependent order;

FIG. 2D is a block diagram of the MPEG hierarchy;

FIG. 3 is a block diagram of an exemplary packetized video elementarystream;

FIG. 4 is a block diagram of an exemplary transport packet;

FIG. 5 is a block diagram of a decoder in accordance with an embodimentof the present invention;

FIG. 6 is a flow diagram describing detection of erroneous PTS valuesfor NTSC displays in accordance with an embodiment of the presentinvention; and

FIG. 7 is a flow diagram describing detection of erroneous PTS value forPAL displays in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the following embodiments are described in the context of theMPEG-2 standard, it should be noted that the present invention is notlimited to the MPEG-2 standard and is also applicable in other contexts.

Referring now to FIG. 1, there is illustrated a block diagram of anexemplary video-on-demand system 100 for transporting a video program toa remote display device 110. The video-on-demand system 100 includes avideo server 105 and a video library 110. The video server 105 iscapable of selecting a particular video program from the video library110 and transmitting the video program as a bit stream to a particulardestination using a communication channel. The video program includesboth a video stream component and audio stream component. Due tobandwidth and memory considerations, the video program is stored andtransmitted in a compressed format known as MPEG-2. Pursuant to MPEG-2,the video is represented by a video elementary stream and the audio isrepresented by an audio stream. At the destination, the video program isreceived and displayed on a display device. Accordingly, with referenceto FIG. 1, the bit stream is received by a decoder 115 which decodes thecomponent video and audio, resulting in a video and audio stream. Thevideo stream is then displayed on a display device 120, and the audiostream is sent to a speaker 125.

The particular video program that is selected by the video server 105can be determined by a user associated with the decoder 115. A user cancause the decoder 115 to transmit a signal to the video server 105indicating a particular video program. Responsive thereto, the videoserver 105 selects the video program from the video library 110 andtransmits the video program to the decoder 115. Additionally, the videoserver 105 provides various modes of operation, known as trick modes,wherein the user at decoder 115 can control the flow of the video, usingvideo cassette recorder-like functions. The trick modes allow the userto fast forward, rewind, jump, and pause the video.

As noted above, the video program is stored as a component videoelementary stream and audio elementary stream. Referring now to FIG. 2,there is illustrated a block diagram describing the compression of avideo in accordance with the MPEG-2 standard. In FIG. 2A, there isillustrated an exemplary video stream. A video stream is a series ofinstantaneous images associated with particular time intervals. Eachimage is associated with a frame 205(1). . . . 205(n). A frame 205 is atwo-dimensional grid of pixels, wherein each pixel in the gridcorresponds to a particular spatial location of the image at theparticular time interval. In some cases, the frames 205 can comprise twofields, wherein the fields are associated with adjacent time intervals.

Pursuant to MPEG-2 the frames 205(1). . . . 205(n) are encoded usingalgorithms taking advantage of both spatial redundancy and/or temporalredundancy, thereby resulting in a video elementary system. Thealgorithms taking advantage of spatial redundancy utilize discretecosine transformation (DCT), and quantization, to reduce the amount ofdata required to code each picture.

The algorithms taking advantage of temporal redundancy use motioncompensation based prediction. With frames 205 which are closelyrelated, it is possible to accurately represent or “predict” the data ofone picture based on the data of a reference field 205, provided thetranslation is estimated. Pictures can be considered as snapshots intime of moving objects. Therefore, one field 205 can be associated witha displacement of another field 205.

Pursuant to the MPEG Standard, many frames 205 are predicted fromanother reference frame(s) 205. A two-dimensional motion vector(s)represents the vertical and horizontal displacement between the field205 and the reference frame(s) 205. The difference between the frame 205and the reference frame 205 is the prediction error. The predictionerror can be encoded in the DCT domain using a small number of bits forrepresentation.

Referring now to FIG. 2B, there is illustrated an exemplary blockdiagram of encoded frames, known as pictures I₀, B₁, B₂, B₃, P₄ . . . ,I_(n), B_(1+n), B_(2+n), B_(3+n), and P_(4+n), representing the videostream in FIG. 2A. The data dependence of each picture is illustrated bythe arrows. For example, picture B₃ is dependent on pictures I₀ and P₄.Pictures coded using temporal redundancy with respect to earlierpictures of the video sequence are known as predicted pictures (orP-pictures), for example picture P₄. Pictures coded using temporalredundancy with respect to earlier and later pictures of the videosequence are known as bi-directional pictures (or B-pictures), forexample, pictures B₁, B₂, B₃. Pictures which are not coded usingtemporal redundancy are known as I-pictures, for example I₀.

The foregoing data dependency among the pictures requires decoding ofcertain pictures prior to others. Additionally, the use of laterpictures as reference pictures for previous pictures, requires that thelater picture is decoded prior to the previous picture. As a result, thepictures cannot be decoded in temporal order. Accordingly, the picturesare transmitted in data dependent order. Referring now to FIG. 2C, thereis illustrated a block diagram of the pictures in data dependent order.

The pictures are further divided into groups known as groups of pictures(GOP). Referring now to FIG. 2D, there is illustrated a block diagram ofthe MPEG hierarchy. The pictures of a GOP are encoded together in a datastructure comprising a GOP Header 240 a and a GOP Payload 240 b. The GOPHeader 240 a includes a broken link bit 245. The broken link bit 245permits play of the GOP in midstream. The GOP Payload 240 b stores eachof the pictures in the GOP in data dependent order. GOPs are furthergrouped together to form a sequence. The GOPs of the sequence areencoded together in a data structure comprising a sequence header 250 aand a sequence payload 250 b.

The video is represented by any number of sequences. The encodedsequences together form the video elementary stream 260. Responsive to auser selection, the video server 105 packetizes the video elementarystream 260 and an associated audio elementary stream. Each videoelementary stream packet is associated with a particular picture. Thepacketized video elementary stream 260 packets are then placed intransport packets. The transport packets are transmitted over thecommunication channel to the destination.

Referring now to FIG. 3, there is illustrated a block diagram describingan exemplary video elementary stream packet 300. The video elementarystream packet 300 includes a packet header 305, a basic extension 310,an optional PTS field 315, an optional DTS field 320, and a payload 325.The header 305 comprises five bytes, followed by the basic extension 310which comprises three bytes. The basic extension 310 includes a PTS flag310 a, and a DTS flag 310 b. In the case where the PTS flag 310 a isset, the PTS field 315 comprising five bytes is appended to the basicextension. In the case where the DTS flag 310 b is set, the DTS field315 comprising five bytes is appended to the basic extension. Thecombination of the DTS flag 310 b set only is not permitted. The PTSfield 315 stores a 33-bit PTS, while the DTS field 320 stores a 33-bitDTS.

Referring now to FIG. 4, there is illustrated a block diagram of anexemplary transport packet 400. The transport packet 400 includes aheader 400 a, an adaption field 400 b, and a payload 400 c. Theadaptation field 400 b includes a discontinuity indicator flag 405, aprogram clock reference flag 410, and can include a program referenceclock field 415.

The program reference clock value 415 is used to synchronize the systemtime clock of the video server 105 with the MPEG decoder 115. Each videoprogram is associated with a particular system time clock.Synchronization of the system time clock at the video server 105 and theMPEG decoder 115 is achieved by sampling the system time clock when thetransport packet is in a constant delay part of the transmission path.The sampled system time clock is inserted as the program reference clockvalue 415 every 100 ms. The existence of the program reference clockfield 415 in the transport packet 400 is indicated by setting theprogram clock reference flag 410.

As noted above, the user can cause the video program to pause or fastforward using a trick mode. During a trick mode, the normal stream offrames 205 is interrupted. In the case of fast-forward, a fast-forwardfile associated with the video program is assembled into transportpackets and transmitted to the user. The fast-forward file is also avideo elementary stream, but contains only the I-pictures from the videoelementary stream representing the video program. Since usually anI-frame is inserted at intervals of 15 pictures of the video elementarystream displaying the fast forward file appears like the video programis being played at a fast speed. When play is resumed, the videoelementary stream representing the video program is again assembled intotransport packets and transmitted. During a pause operation, the videoserver 105 stops transmitting the video transport stream after twoB-pictures. When the user returns to play mode, the video transportstream is resumed.

Many of the trick modes involve discontinuities in the play of the videoelementary stream representing the video program. At the transitionpoint, the video server 105 signals the decoder 115, indicating thediscontinuity. The discontinuity is indicated to the decoder 115 becausethe decoder 115 maintains the time base of the video elementary streamrepresenting the video program. During the trick mode, the time base canbe changed. For example, during fast forward, the fast forward file isassociated with a different program reference clock. The discontinuityis generally indicated by transmission of a transport packet 400 withthe discontinuity indicator 405 set, a new program clock referencestored in the program clock reference field 415, and the payload 420containing a video sequence end code.

Many low cost and older video servers 105 fail to transmit theappropriate indicators during trick mode transition. As a result,various error types can occur. For example, a PCR discontinuity erroroccurs when the video server 105 sends a video elementary stream 260with another time base, but fails to set the discontinuity bit 405. Uponreceiving the video elementary stream 260 with the new time base, thedecoder 115 generates PCR discontinuity errors. However, the foregoingPCR discontinuity errors can be avoided by detecting that the differencebetween the received PCR and the STC exceeds a certain programmablethreshold. Responsive, thereto, the time base can be changed to the newPCR.

Referring now to FIG. 5, there is illustrated a block diagram of anexemplary MPEG decoder 115 in accordance with an embodiment of thepresent invention. The MPEG decoder 115 comprises a system demultiplexer505, a digital phase lock-loop 510, a video data buffer 515, a decodercontroller 520, a video decoder 525, and a processor 530. The MPEGdecoder 115 receives a transport stream of MPEG transport packets 400 atthe system demultiplexer 505 from the communication channel. The systemdemultiplexer extracts the PCR 415 and provides the PCR 415 to thedigital phase lock-loop 510. The digital phase-lock loop 510 maintainsthe system time clock for the MPEG decoder 115. The digital phase-lockloop 510 compares the PCR 415 to the system time clock, unless the PCRdiscontinuity bit 405 is set. If the discontinuity bit 405 is set, thedigital phase lock loop 510 updates the system time clock to the PCR415.

If the discontinuity bit 405 is not set, the PCR 415 should correspondwithin a certain margin of error to the system time clock. Normallyduring trick mode transition, the discontinuity bit 405 should be set.However, as noted above, many low cost servers do not set thediscontinuity bit 405 during the transition. During a trick modetransition, the PCR 415 is based on a new time base and is likely to besignificantly different from the system time clock. Accordingly, if thePCR 415 and the system time clock differ by more than a predeterminedthreshold, it is likely that the difference is due to a change in timebase. Accordingly, the digital phase-lock loop 510 transmits aninterrupt to the processor 530, indicating that the PCR 415 and thesystem time clock differ by more than a predetermined threshold.

Responsive thereto, the processor 530 programs the digital phase-lockloop 510 to update the system time clock with the new PCR. When thesystem time clock is changed, the video decoder 525 will automaticallydisable time base management at the decoder control 520 to allowpictures stored in the video data buffer 515 that are still based on theprevious time base to continue to be decoded. While the pictures basedon the previous time base are decoded, the video decoder 525 operates inthe V-synch mode, wherein the pictures are decoded at each verticalsynchronization pulse. Once all pictures based on the previous time baseare decoded and displayed, the video decoder 525 automatically enablestime base management at the decoder control 520.

Alternatively, for decoders 115 which are not able to quickly adapt tothe time base change, the interrupt is handled by disabling the PCR andsystem time clock comparison at the phase-lock loop 510. The processorcalculates the DTS and PTS using the original time base and inserts thecalculated DTS and PTS in the DTS field 320 and the PTS field 315,respectively. The DTS is calculated by incrementing the DTS from theoriginal time base for each picture. The DTS is calculated using thefollowing formula:DTS(n+1)=DTS(n)+[90 KHz*(2+repeat_first_field)]/(frame_rate*2)The PTS can be calculated from the DTS in a manner well known in theart.

Another problem encountered during trick mode transition is mislabeledtime stamps. In some cases, the PTS 315 and DTS 320 are miscalculated oromitted altogether by the video server 105. Therefore, the picturesduring the transition are not decoded and presented at the appropriatetimes. The PTS 315 and DTS 320 miscalculations sometimes occur becausethe pictures are in film mode and the video server 105 does not adjustthe calculations for display using the National Television StandardCommittee (NTSC) Standard.

When the pictures are in film mode, each picture is divided into twofields. Each field is displayed at a rate of 60 field/sec. However,because the film mode records only 24 frames per second, only 48 fieldsper second are produced. In order to display the 48 fields in propersynchronization, one out of every four fields is repeated. Themiscalculated PTS 315 and DTS 320 can be corrected by the processor 530.In the case where the MPEG decoder 115 detects a transition and isreceiving film mode pictures, the processor 530 checks for the correctPTS value 315 for each picture. If the processor 530 finds an incorrectPTS value, the processor 530 disables the PTS and DTS.

Referring now to FIG. 6, there is illustrated a flow diagram describingdetection of PTS errors for film mode pictures displayed using the NTSCstandard, responsive to detection of a trick mode transition, inaccordance with an embodiment of the present invention. At 605, thevariable B is set equal to the PTS value 315 for the first picture afterthe trick mode transition is detected minus 3003, and RFF is initializedto zero. The variable B is used to store the PTS of the last picture andthe variable RFF is used to store the repeat_first_field parameterextracted from the picture coding extension for the previous picture.For the first picture after the transition, RFF is initialized to zero.At 610, A is set equal to the PTS value 315 for the instant picture.

A determination is made whether the previous picture included a fieldthat was repeated (615). If the last previous picture included a fieldthat was repeated, the PTS for the for the instant picture isincremented from the PTS of the previous picture by the amount of timerequired for display of three frames, instead of two. Using 90 KHztiming, and a display rate of 60 fields/sec, the amount of time is 4504cycles. Accordingly, if the last previous picture included a field thatwas repeated, delta is set to 4504 at 620.

However, if the previous picture did not include a field that wasrepeated, the picture is incremented by the amount of time required fordisplay of two frames, e.g., 3003. Accordingly, if the last previouspicture included a field that was not repeated, delta is set to 3003 at625. At 625, the difference between the PTS 315 for the instant pictureand the PTS 315 for the previous picture is compared to the calculatedincrement, delta. If the difference is not within a certainpredetermined margin of error, e.g., 2, from delta during 625, the PTSis incorrect and the processor disables (at 630) the PTS and DTS byclearing the enable bit in the PES packet header. If the difference iswithin a certain predetermined margin of error from delta during 625,the PTS 315 is correct and 630 is bypassed. At 635, the variable is setto the PTS value 315 of the instant packet and RFF is set to therepeat_first_frame for the instant packet and 610-635 are repeated forthe next picture.

Referring now to FIG. 7, there is illustrated a flow diagram fordetecting PTS errors wherein the pictures are displayed using the PALstandard, in accordance with an embodiment of the present invention. ThePAL standard displays 25 frames per second. At 705, the variable B whichstores the PTS value of the previous picture is initialized for thefirst picture after the transition by setting the variable B to the PTSvalue 315 of the instant packet minus the time for the display of onepicture, e.g., 3600. At 710, the variable A is set to the PTS value 315of the instant picture. At 715, the difference between the PTS value 315of the instant picture and the PTS value 315 of the previous picture iscompared to 3600. If the difference is not within a predetermined marginof error from 3600, the PTS is invalid. Accordingly, the processor 530disables the PTS 315 and DTS 320 by clearing the enable bit in thetransport packet (720). If the difference is within a predeterminedmargin of error from 3600, the PTS is valid and 720 is bypassed. At 725,the variable B is set to the PTS value 315 for the instant packet and710-725 are repeated for the next picture.

Another problem encountered during trick mode transition is theappearance of block artifacts immediately after the transition. This canoccur when the user transitions from fast forward or pause to play. Whenthe user transitions from fast forward or pause to play, the picturesare transmitted starting at a particular picture. In the fast forwardmode, the pictures are transmitted starting from the last displayedI-frame in the fast forward at the point where the user transitionedfrom fast forward to play. In the pause mode, the pictures are startedfrom the last displayed picture at the point where the user paused thevideo. The starting pictures are likely to be in the middle of a GOP, asopposed to the start of the GOP. This can cause a problem when decodingB-pictures because the B-pictures might be predicted from pictures whichwere prior to the starting point. Therefore, after the transition, thevideo server 105 indicates to the MPEG decoder 115 that the pictureswill start from the middle of a GOP. This is indicated by setting a GOPbroken link bit in the first GOP. Setting the GOP broken link bit 245causes the MPEG decoder 115 to discard the first two B-pictures in thefirst GOP. If the video server 105 fails to set the GOP broken link bit,the block artifacts can appear on the display.

The blocky artifacts which occur when the video server 105 fails to setthe GOP broken link bit 245 are avoided because the processor 530discards the first two B-frames after detecting a transition, regardlessof whether the GOP broken link bit 245 is set.

Additionally, the processor 530 can detect erroneous sequence end codesby determining if the sequence end code is accompanied by a time basechange. If the sequence end code is not accompanied by a time basechange, the sequence end code is determined to be erroneous.

Based on the foregoing, those skilled in the art should now understandand appreciate that the foregoing advantageously provides a techniquefor handling common errors which occur during trick mode transitioning.The techniques presented herein can be used with preexisting videoservers with requiring modifications to the video servers.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt particular situation or material tothe teachings of the invention without departing from its scope. Forexample, the flow diagrams of FIGS. 6 and 7 can be implemented as aseries of instructions residing in a memory for execution by aprocessor, such as processor 530. Therefore, it is intended that theinvention not be limited to the particular embodiment(s) disclosed, butthat the invention will include all embodiments falling within the scopeof the appended claims.

1. A method for receiving packets, said method comprising: receiving apacket, wherein the packet indicates a continuous time base; comparing atime stamp of the received packet to a clock; and setting the clock tocorrespond to the time stamp, if the clock and the time stamp differ bymore than a predetermined threshold; and storing one or more packetsreceived before the received packet in a decoder buffer; and decodingthe one or more packets received before the received packet in avertical synchronization pulse mode with a video decoder.
 2. The methodof claim 1, wherein receiving the packet further comprises: examining aprogram clock reference discontinuity indicator in the packet.
 3. Themethod of claim 1, wherein comparing the time stamp further comprises:comparing a program clock reference stored in said received packet tothe clock.
 4. The method of claim 1, further comprising: disabling timemanagement responsive to setting the clock; and decoding one or morepackets received before the received packet.
 5. The method of claim 4,further comprising: enabling time management responsive to decoding theone or more packets received before the received packet; and decodingpackets received after the received packet using a time basecorresponding to the time stamp.
 6. A decoder for receiving packets,said decoder comprising: a demultiplexer for receiving a packet, whereinthe packet indicates a continuous time base; a phase-lock loop forcomparing a time stamp of the received packet to a clock andtransmitting an interrupt if the clock and the time stamp differ by morethan a predetermined threshold; a processor for setting the clock tocorrespond to the time stamp, responsive to said interrupt; and adecoder buffer for storing one or more packets received before thereceived packet; and a video decoder for decoding the one or morepackets received before the received packet in a verticalsynchronization pulse mode.
 7. The decoder of claim 6, wherein thedemultiplexer extracts the time stamp from the received packet andprovides the time stamp to the phase-lock loop.
 8. The decoder of claim6, further comprising: a decoder controller for time management using atime base corresponding to the time stamp; and wherein the decoderdecodes packets received after the received packet using the time basecorresponding to the time stamp, responsive to decoding the one or morepacket received before the received packet.
 9. The method of claim 1,wherein setting the clock to correspond to the time stamp furthercomprises setting the clock to equal the time stamp.
 10. The decoder ofclaim 6, wherein setting the clock to correspond to the time stampfurther comprises setting the clock to equal the time stamp.